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Four years ago, the unexpected discovery in the clouds of Venus of a gas that means life on Earth – phosphine – faced controversy, earning criticism in subsequent observations that failed to match his findings.

Now the same team behind that discovery is back with more observations, presented for the first time on July 17 at a meeting of the Royal Astronomical Society in Hull, England. Ultimately, they will form the basis of one or more scientific studies, and that work has already begun.

The data, the researchers say, contains even stronger evidence that phosphine is present in the clouds of Venus, our closest planetary neighbor. Sometimes called Earth’s evil twin, the planet is similar in size to ours, but has surface temperatures that can melt lead and clouds made of corrosive sulfuric acid.

The work benefited from a new receiver installed on one of the instruments used for the observations, the James Clerk Maxwell Telescope in Hawaii, giving the team more confidence in their findings. “There’s also a lot more to it than the data itself,” said Dave Clements, reader in astrophysics at Imperial College London.

“We had three monitoring campaigns and in just one run we got 140 times more data than the original detection,” he said. “And what we have so far shows that we have phosphine discoveries again.”

A separate team, including Clements, presented evidence for another gas, ammonia.

“This is perhaps more significant than the discovery of phosphine,” he added. “We’re a long way from saying that, but if there is life on Venus that produces phosphine, we have no idea why it’s producing it.” However, if there is ammonia-producing life on Venus, we have an idea why it might want to inhale ammonia.

On Earth, phosphine is a foul-smelling, toxic gas produced by decaying organic matter or bacteria, while ammonia is a pungent-smelling gas that occurs naturally in the environment and is also produced primarily by bacteria at the end of the process of decomposing plant and animal waste .

“Phosphine was found in Saturn’s atmosphere, but this is not unexpected because Saturn is a gas giant,” Clements said. “There’s an awful lot of hydrogen in its atmosphere, so any hydrogen-based compounds like phosphine or ammonia are what dominate there.”

Rocky planets like Earth, Venus, and Mars, however, have atmospheres where oxygen dominates the chemistry because they did not have enough mass to retain the hydrogen they had when they originally formed, and that hydrogen escaped.

Therefore, finding these gases on Venus is unexpected. “By all normal expectations, they shouldn’t be there,” Clements said. “Phosphine and ammonia have been proposed as biomarkers, including of exoplanets. So finding them in the atmosphere of Venus is interesting on that basis as well. When we published the phosphine findings in 2020, understandably, it was a surprise.”

Subsequent studies disputed the results, suggesting that the phosphine was actually simple sulfur dioxide. Data from instruments other than those used by Clements’ team — such as the Venus Express spacecraft, NASA’s infrared telescope and the now-defunct SOFIA airborne observatory — also failed to replicate the phosphine findings.

But Clements said his new data, coming from the Atacama Large Millimeter/submillimeter Array, or ALMA, ruled out the possibility that sulfur dioxide was a contaminant, and that the lack of phosphine from other observations was due to timing. “It turns out that all of our observations that detected phosphine were taken when the atmosphere of Venus moved from night to day,” he said, “and all the observations that did not detect phosphine were taken when the atmosphere has moved from day to night.”

During the day, ultraviolet light from the sun can disrupt molecules in Venus’ upper atmosphere. “All the phosphine is baked, and that’s why you don’t see it,” Clements said, adding that the only exception is the Stratospheric Observatory for Infrared Astronomy, which makes observations at night. But further analysis of this data by Clements’ team revealed faint traces of the molecule, strengthening the theory.

Clements also pointed to unrelated research by a group led by Rakesh Mogul, professor of chemistry and biochemistry at California State Polytechnic University, Pomona. Mogul reanalyzed old data from NASA’s large Pioneer Venus probe, which entered the planet’s atmosphere in 1978.

“It showed phosphine in the clouds of Venus at about a part-per-million level, which is pretty much what we’re finding,” Clements said. “So it’s starting to hang together, but we still don’t know what’s producing it.”

Using data from the Pioneer Venus Large Probe, the team led by Mogul published in 2021 “a convincing case for phosphine deep in the cloud layer (of Venus),” Mogul confirmed in an email. “To date, our analyzes remain unchallenged in the literature,” said Mogul, who was not involved in Clements’ team’s research. “This is in stark contrast to the telescopic observations, which remain controversial.”

Ammonia on Venus will make an even more surprising discovery. Presented at the talks in Hull by Jane Greaves, professor of astronomy at Cardiff University in the UK, the findings will form the basis of a separate scientific paper using data from the Green Bank Telescope in West Virginia.

The clouds on Venus are made of droplets, Clements said, but they are not water droplets. They contain water, but also so much dissolved sulfur dioxide that they turn into extremely concentrated sulfuric acid, a highly corrosive substance that can be fatal to humans in severe exposure. “It’s so concentrated that, as far as we know, it wouldn’t be compatible with any life on Earth that we know of, including extremophilic bacteria that like very acidic environments,” he said, referring to organisms that are capable of to survive extreme environmental conditions.

However, the ammonia in these acid droplets can act as a buffer for the acidity, lowering it to a low enough level that some known terrestrial bacteria can survive in it, Clements added.

“The exciting thing behind this would be if some kind of microbial life was producing the ammonia, because that would be a neat way of regulating its own environment,” Greaves told the Royal Astronomical Society talks. “It would make the environment much less acidic and much more survivable, to the point where it’s as acidic as some of the most extreme places on Earth — so it’s not totally crazy.”

In other words, the role of ammonia is easier to explain than that of phosphine. “We understand why ammonia can be beneficial to life,” Clements said. “We don’t understand how ammonia is made, just like we don’t understand how phosphine is made, but if ammonia is there, it will have a functional purpose that we can understand.”

However, Greaves cautioned that even the presence of phosphine and ammonia would not be evidence of microbial life on Venus because so much information is missing about the state of the planet. “There are many other processes that could go on, and we just don’t have the ground truth to say whether that process is possible or not,” she said, referring to the hard evidence that can only come from direct observations from inside the atmosphere of the planet.

One way to make such observations would be to convince the European Space Agency to include some instruments aboard the Jupiter Icy Moons Explorer — a probe on its way to the Jupiter system — when it flies by Venus sometime next year. But even better data would come from DAVINCI, an orbital and atmospheric probe that NASA plans to launch to Venus in the early 2030s.

Scientifically, the new data on phosphine and ammonia are intriguing but call for cautious optimism, said Javier Martin-Torres, a professor of planetary sciences at the University of Aberdeen in the United Kingdom. He led a study published in 2021 that disputed the phosphine findings and postulated that life was not possible in the clouds of Venus.

“Our paper highlighted the harsh and seemingly inhospitable conditions in Venus’ atmosphere,” Martin-Torres said in an email. “The discovery of ammonia, which could neutralize sulfuric acid clouds, and phosphine, a potential biosignature, challenges our understanding and suggests that more complex chemical processes may be involved. It is critical that we approach these findings with careful and thorough scientific investigation.

The findings open up new avenues of research, he added, but it’s essential to treat them with a healthy dose of skepticism. While the discovery of phosphine and ammonia in the clouds of Venus is exciting, it is only the beginning of a longer journey to unravel the mysteries of that planet’s atmosphere, he said.

Scientists’ current understanding of Venus’ atmospheric chemistry cannot explain the presence of phosphine, said Dr Kate Patel, a lecturer in the Department of Physics and Astronomy at University College London. “It is important to note that the team behind the phosphine measurements does not claim to have discovered life on Venus,” Patel said in an email. “If phosphine is indeed present on Venus, it could mean life, or it could mean there’s atmospheric chemistry on Venus that we don’t yet understand.”

The discovery of ammonia would be exciting if confirmed, Patel added, because ammonia and sulfuric acid shouldn’t be able to co-exist without some process — whether volcanic, biological or something yet to be thought of — driving production of ammonia itself.

She emphasized that both results are only preliminary and require independent confirmation, but they make upcoming missions to Venus such as the Jupiter Icy Moons Explorer and DAVINCI intriguing, she concluded.

“These missions may provide answers to questions raised by recent observations,” Patel said, “and will certainly give us fascinating new insights into our nearest neighbor’s atmosphere and its ability to harbor life.”